Combine the chocolate and oil in a microwaveable bowl. Microwave on high power for 1 minute, or until the chocolate pieces are glossy and lose their shape. Stir until smooth.

Combine the flour, cocoa, flaxseed, baking powder, and salt on a large sheet of waxed paper. Stir to blend.

Beat the egg and egg white with a fork until smooth in a mixing bowl. Add the sugar and mix, breaking up any lumps. Add the tomato puree, vanilla exract, and chocolate mixture. Stir until smooth. Add the dry ingredents, mixing until thorougly blended. Dollop the batter into the prepared pan. Scatter the walnuts evenly over the top, pressing lightly with a spatula to embed them partially in the batter.

Bake for 20 to 25 minutes, or until a wooden pick inserted into the center has moist crumbs clinging to it. Do not overbake. Cool for 10 minutes before cutting into 16 pieces. Dust lightly with confectioner's sugar, if desired.

MAKE IT AHEAD: Brownies taste best served warm. To keep them longer, cut and place them on a tray. Freeze for several hours, then wrap them individually in plastic wrap. Freeze for up to 3 months.

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